I was just wondering, are there any modifications that can be done to an HK USP9 magazine (or any HK pistol magazine, for that matter) to increase the magazine capacity and still have it fit flush with the mag well?

For example, with SIG 15 round 9mm x 19 magazines, the magazine capacity can be increased to 17 rounds by switching the standard follower with a low capacity follower and the magazine still fits flush with the mag well (as per vanilla_gorilla).  Are you aware of a similar modification for HK pistol magazines?

Thanks for the help.

Regards,

Justin

I read somewhere that you can use 40 mags in a 9 and it will work...

Yes, that will work but you gain no more than if using the normal full caps.  However, if you are in a 10 round limit state the use of a .40 mag in the 9mm will gain you two or three rounds (Compact-2, full size-3).  
There is nothing that I know of that will gain you extra ammo without affecting function and still have the mag fit flush.  After the ban ended I took my 10 round mags in full size and Compact, chopped the top off one and had it welded to the bottom of another and got mag capacity up from 10 to 17 rounds, but they stick way the hell out of the bottom.  For full size use a 170mm STI spring, Compact a 140 mm STI spring.